Yes, it’s somewhat foetal but I doubt that was what Flandrin had in mind. It’s related in part to another painting he did reminiscent of Greek pottery and sculpture. I think he was aiming for that kind of Classical simplicity but inadvertently ended up hitting something that gay men find attractive. And that attraction probably relates to it being a masculine figure in a more vulnerable posture than you usually see.

If you look at the original Evolution of an Icon post you’ll see a recent comment pointing to some pictures by Maxfield Parrish with a similar pose but female figures.

Well – to try to bring some answer to your question as to why so many artists use this pose – it is a very chaste pose for a nude, so it would appeal to a model who is not used to taking off his or her clothes (the egg like position acting as some type of mental protection for the model). And the exposed back allows nice light plays too.
And for sketching, because it is a compact pose, it brings the model to fit in nicely into A size paper, making most of the surface available.